A locking key of this description is the subject matter of my prior U.S. Pat. No. 4,432,218. According to that prior patent, a bit-carrying shank and an operating head of the key are relatively rotatable through 90.degree. or 180.degree.. The shank has an extension in the form of a bolt which is axially threaded into the head and carries a terminal element on a free end projecting from that head. A pair of transverse shoulders respectively formed on the shank and on the terminal element confront a pair of lands on the head, the separation of the shoulders exceeding that of the lands whereby a gap exists between one shoulder and the corresponding land in either limiting position of relative rotation. The pitch of the bolt thread is so chosen that the gap shifts from one shoulder to the other upon a relative rotation by the desired fraction of a turn, whereupon the shank is positively entrained by the head when the latter continues to be rotated in the same sense.
While the key of my prior patent operates satisfactorily in most instances, it may happen that the exertion of excessive force in the opening or the closing of the lock causes one of the lands to become wedged against the associated shoulder so as to be disengageable therefrom only by a torque exceeding that which is needed to reverse the position of the lock. In that case the relative angular position of the head and the shank, which may be marked by distinctive symbols or colors, no longer correctly indicates to the user the lock-shifting operation last performed.